Android Cheat Sheet

Author: bluelul.com

Install

Ubuntu

1. Download Android Studio at Android Homepage.
2. Extract to any folder you want, i.e. {installation home}
3. Open terminal at {installation home}

./bin/studio.sh

4. Follow instruction to install Android Studio (custom option recommended)

Note

• If your computer supports GPU (especially NVIDIA), install GPU driver for optimizing AVD (Android Virtual Device) perfomance.

sudo ubuntu-drivers autoinstall

• If you can not start Android Emulator, with error /dev/kvm device permission denied, then run these commands:

sudo apt install qemu-kvm
sudo adduser $USER kvm
sudo chown $USER /dev/kvm

Customize

Theme

1. Open File → Settings → Plugins in Android Studio
2. Install theme you want. Recommend: One Dark Theme, Hiberbee Theme, Material Theme

Tweak

Keep Android Emulator always on top

On active widow emulator, press ALT + SPACE, select always on top

Logcat monitor tool (Ubuntu)

• Colorize logcat:

sudo snap install logcat-colorize
adb shell logcat | logcat-colorize

Colorize adb shell

Colorize adb shell

• Only supported for devices with file-system writable
• Copy all text below and paste in one shot to adb shell

echo >> /system/etc/mkshrc
echo -e "# Colorize: bluelul.com\nif (( USER_ID )); then PS1='$'; else PS1='#'; fi" >> /system/etc/mkshrc
echo -e "PS4='[\$EPOCHREALTIME] '; PS1='\${|" >> /system/etc/mkshrc
echo -e "local e=\$?" >> /system/etc/mkshrc
echo -e "(( e )) && REPLY+=\"\033[01;31m\$e\033[00m|\"" >> /system/etc/mkshrc
echo -e "return \$e" >> /system/etc/mkshrc
echo -e "}\033[01;33m\$HOSTNAME\033[00m:\033[01;34m\${PWD:-?}\033[00m'\"\$PS1 \"" >> /system/etc/mkshrc
echo >> /system/etc/mkshrc
echo "alias ls='ls --color'" >> /system/etc/mkshrc
echo "alias ll='ls -l --color'" >> /system/etc/mkshrc
echo "alias lah='ls -lah --color'" >> /system/etc/mkshrc

• Exit adb shell (Ctrl-D) and access adb shell again to update changes

Code snippet / Live Template

Install template

1. Copy all content of bluelulAndroid.xml to clipboard
2. Open File → Settings (Ctrl + Alt + S) in Android Studio, then Editor → Live Templates
3. Press + button on the right (or press Alt + Insert) and choose Template Group
4. Input AndroidBluelul or any name you want
5. Press Ctrl + V to add all copied templates to this group

Log

• logt : assign TAG
• logi : log info
• loge : log error
• logd : log debug

Bind layout

• find : bind XML component in layout to Java object
• findquick : bind XML component in layout to Java object without declaring

Button

• btnclick : set OnClick Listener to button object (non-lambda style)
• btnclickquick : set OnClick Listener to button object (lambda style + non pre-declared)
• button.setOnClickListener(v -> {}) : set OnClick Listener to button object (lambda style)

TextView

• textView.setText(valueStr) : change content of TextView object

EditText

• editText.setText(valueStr) : change content of TextView object
• editText.length() : get text length
• editText.setError("show error") : trigger error in EditText object
• textchangewatch : handle text change event

TextInputLayout

• editText.setText(valueStr) : change content of TextView object
• editText.length() : get text length
• editText.setError("show error") && editText.setErrorEnabled(true) : show error in TextInputLayout object
• editText.setErrorEnabled(false) : hide error in TextInputLayout object
• textchangewatch : handle text change event

Radio

• radioGroup.getCheckedRadioButtonId() : return Resource ID of checked radio button (e.g. R.id.radioButton), return -1 if no button checked

Activity

• swact : switch to another activity
• loadintent : load input argument of activity after being created
• overridePendingTransition(R.anim.trans_left_in, R.anim.trans_left_out) : swipe left to new activity, run after triggering activity
• overridePendingTransition(R.anim.trans_right_in, R.anim.trans_right_out) : swipe right to previous activity, run after triggering activity

Fragment

Note: layout-related code should be placed on onViewCreated method, and use view.findViewById instead of this.findViewById

• fragreplace : Replace fragment to frame layout
• fragadd : Add (overlay) fragment to frame layout

Intent

• call : dial a phone number
• sms : compose a SMS message to a phone number
• web : browse a website
• mail : compose an email

Clipboard

• clipman : declare ClipboardManager object
• clipcopytext : copy plain text to clipboard
• clippastetext : paste plain text from clipboard
• clipcopyintent : copy intent to clipboard
• clippasteintent : paste intent from clipboard and run intent
• clipcopyrawuri : copy raw uri to clipboard
• clippasteuri : paste uri from clipboard

URI

• res2uri : parse URI from Resource ID

Toast

• toast : make new toast
• toast.setGravity(Gravity.CENTER | Gravity.TOP, 0, 0) : change toast position

Snackbar

• snack : make snackbar with button
• snacktext : make new snackbar (for notification)

Layout tweak

• Fill white color to icon of layout component: app:tint="@color/white"
• Round button: app:cornerRadius="100dp"
• Only-contain-icon button: app:iconGravity="textStart" app:iconPadding="0dp"
• List-item can't be click after contain buttons: add android:focusable="false" to all buttons and clickable components
• Hide spell check but keep autocomplete for EditText: android:inputType="_current_input_type_|textPersonName"

Quickcode template

Templates are stored in /quickcode folder

• double back to exit : press back button 2 times to exit app
• floating action button expandable : press the (+) fab to expand more fabs
• listview simple : make simple listview (only text, single choice, multiple choice)
• listview card : make card listview (1 item contains: avatar image, main name text, sub name text)
• listview order : make food order listview (1 food item contains: image, name, description, price, count number, add button, remove button (long press to empty all))
• landview linearlayout adapt : re-arrange layout when phone orientation change between landscape (horizonal) and portrait (vertical) by changing orientation of linearlayouts

Protection Level

Normal

• Default protectionLevel of permissions with low risk to other apps, system or user
• Automatic granted to requested app, without prompting the user
• User allows permission before installing app

Dangerous

• Higher than Normal
• Permissions with high risk to private user data accessing or device controlling, e.g. get location, write contact, answer phone call, etc.
• Only granted after asking the user

Privileged

• Higher than Dangerous
• Only granted to apps installed under the folder /system/priv-app
• Apps installed in /system/priv-app will have Privileged level
• Read more about system app behaviour

How to elevate

• Method 1: hot install
  • Get the right to write to read-only /system folder: the usual way
  • Install app.apk to Android OS:

  adb install app.apk

  • Upload app.apk to device/emulator:

  adb push app.apk /system/priv-app

  • Reboot device/emulator:

  adb reboot

  • Note: to uninstall, first remove /system/priv-app/app.apk, then reboot and uninstall as normal app
• Method 2: build with OS
  • Place app in /system/priv-app
  • Build app along with your custom Android OS
  • Flash this OS to your device

Signature

• Higher than Privileged
• Only granted to app signed with OEM system certificate
• Elevate an app with Signature right by signing this app with platform signature, issued by device manufacturer (OEM)
• Read more about system app behaviour

How to elevate

• Method 1: sign by public & private key (the easiest way)
  • Get platform.x509.pem (public key) and platform.pk8 (private key)
    • For Android Emulator: download from aosp-mirror, or get files directly from /sign folder in this repo
    • For custom device: get files from build/target/product/security/ in android source code folder
  • Install apksigner 0.9 with apksigner_0.9-1_all.deb file in /sign folder in this repo

  sudo apt install -y fastjar jarwrapper
  sudo dpkg -i apksigner_0.9-1_all.deb

  • Sign your app with given public & private key:

  apksigner sign --key platform.pk8 --cert platform.x509.pem --out `pwd`/sign.apk unsign.apk

• Method 2: sign by keystore
  • Get platform.x509.pem (public key) and platform.pk8 (private key), as in Method 1
  • Get keytool-importeypair from github repo, or from /sign folder in this repo
  • Generate your keystore file

  sudo chmod 777 keytool-importkeypair.sh
  ./keytool-importkeypair.sh -k key.keystore -p android -pk8 platform.pk8 -cert platform.x509.pem -alias platform

  • Grant an App the System Uid (Optional): in the tag in AndroidManifest.xml file, add the attribute android:sharedUserId="android.uid.system"
  • Use keystore file to sign your app in one of below ways:
    • On Android Studio, clicking on 'Build--> Generate Signed Bundle / APK' (alias: platform, password: android)
    • Install apksigner (as in Method 1), then run this command:

    apksigner sign --ks key.keystore --ks-pass pass:android --out `pwd`/sign.apk unsign.apk

    • On Android Studio, using signingConfig in your app's .gradle

    signingConfigs {
        config {
            storeFile file("key.keystore")
            storePassword 'android'
            keyAlias 'platform'
            keyPassword 'android'
        }
    }
    
    buildTypes {
        release {
            signingConfig signingConfigs.config
        }
    }

• Method 3: build with OS
  • Place app in /system/priv-app to prevent being uninstalled by user (optional, but prefer)
  • Add LOCAL_CERTIFICATE := platform to the app's Android.mk
  • Build app along with your custom Android OS
  • Flash this OS to your device

SignatureOrSystem

• Old synonym for Signature|Privileged, deprecated in API level 23
• Accessed by signed app or privileged app
• The easiest way to achieve is by elevating privileged right with Method 1 (hot install to /system/priv-app)

System

• Old synonym for Privileged, deprecated in API level 23

System app vs User app

• System apps are apps installed in /system/priv-app and /system/app in Android device file system
• User apps are apps installed from Play Store, adb install, or directly installed by user from apk files, located in /data in Android device file system
• Apps installed in /system/priv-app will have Privileged protection level
• Apps installed in /system/app are the same as User apps but can not uninstalled by user. Therefore manufacturer's default apps (bloatware) are usually installed in /system/app
• Signed apps are apps signed by platform signature, issued by device manufacturer

App management from Setting/Apps

Install method	Can be uninstalled	Can be disabled
User app	yes	no need
Signed User app	yes	no need
System app	no	yes
Signed System app	no	no

Storage

System app is stored in read-only /system location, lead to:

• System apps can be updated just like user apps, but the update is never integrated into the original rom --> able to roll back to the version that existed when the rom was first installed
• System apps cannot be moved to SD and always take up some internal space --> to get more user app space, uninstall updates of system apps, instead of these apps themselves
• System apps cannot be deleted in a full factory reset, unlike user apps

Writing on Android Emulator file system

• Start emulator with -writable-system to modify system files:

cd ~/Android/Sdk/emulator
./emulator -list-avds
./emulator -writable-system -avd Nexus_4_API_27 -no-snapshot-load -qemu

• Leverate permission to modify system:

adb root

# For Android 8
adb disable-verity
# For Android 10 emulator only
adb shell avbctl disable-verification

adb reboot
adb root
adb remount

Concept

Life Cycle

Activity

Create
|------------- finish data binding, allocate resources
Start
|------------- visible to user
Resume
|-------------------- running
Pause
|------------- freezing: transition to next activity , new app partially cover
Stop
|------------- hiding: dive into Back Stack , Recent App pressed , Home pressed, new app fully cover
Destroy
|------------- free all resources: pop out of Back Stack , kill app

Activity vs Fragment

Simple Lifecycle

Full Lifecycle